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We have a 400 amp service and with a 200 amp panel that feeds two sub panels and a 150 amp panel inside the main box that is currently unused. I'm looking to run that 150amp service to our shop which is about 220ft away 230ft for extra wiggle room. I've been trying to do some research and calculations on what wire I would need to run that underground in conduit with aluminum wire and using different calculators/methods seems to give me similar but different results. One electrician told us 4/0 MHF would work but another said 2/0 minumum. Any guidance would be appreciated. if it makes any difference we plan on running a couple 230v welders, 240v compressor shop tools, lights etc.

Thanks

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    You don't have to run 150A if you aren't going to use 150A. So figure out how much you really expect to use, either anecdotally or (preferably, but a bit hard for a workshop) an official load calculation. Simple rough calculation: Add up all tools that might be used at the same time, add 25% on anything needing a continuous load, add HVAC (if any), throw in 20A for lights, etc. Keep in mind that calculation is normally @ 240V - so 120V machines count 1/2. Then based on what that result is, decide whether 90A or 100A or 125A or 150A, and then determine the size of aluminum wire to use.
    – manassehkatz-Moving 2 Codidact
    Commented Aug 22, 2023 at 16:04
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    @manassehkatz-Moving2Codidact Let's assume 150A since that's what the electrician wanted to install based on the appliances, I told him about
    – Syrus Yeung
    Commented Aug 22, 2023 at 16:58

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Three issues to consider:

  • Load Calculation

This is how you determine how much power you need to provision to your workshop. But in addition, it also tells you how much of your existing service is available to send to the workshop. You really need to have multiple load calculations:

  • Entire service (except the new workshop). Subtract from your 400A feed to figure out how much is available.
  • Each existing panel (main panel or subpanel) to determine whether that panel is provisioned properly and whether there is any excess available.

In this specific situation, you have 400A total. If you are only actually using 250A (or less) then you are fine at that level. However, you are feeding (as is typical) this 150A workshop subpanel off of an existing 200A panel. If that 200A panel is already serving 50A or more of loads (again, based on a load calculation, not based on "add up breaker handles" (which would be too high) and not based on "hunches" (which would likely be too low), then you can't provision 150A to feed a subpanel from there. If it is using, for example, 125A, then you could provision 75A.

  • Wire Size

This is actually quite straightforward. See this chart:

  • 3/0 Aluminum = 155A. That's the smallest size you can use for 150A.

  • Voltage Drop

230 feet, 3/0 aluminum, 150A @ 240V = 3.89%. That is slightly higher than the typical "3% or less" but not by much, and at 120A (80%, which is all you should really be at most of the time) only 3.11%. Where the number gets high (6% or more!) is if a lot of the loads are 120V loads. I doubt that's the case here, but just something to keep in mind, particularly if someone says "over x feet, better upsize for voltage drop".

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